1. Field of the Invention
This invention relates to solvent extraction of a natural gas stream with selected physical solvents to produce C.sub.2 +hydrocarbon products.
2. Review of the Prior Art
U.S. Pat. No. 2,782,141 describes a process for dehydrating a wet natural gas with ethylene glycol, front end cooling to remove all C.sub.4 +components and part of the propane, absorption with a refrigerated lean oil at 450 psig, flashing the rich oil to a pressure of about 60 psig, and dehydrating and then refrigerating the lean gas from the absorption zone to remove C.sub.5 +condensate utilized from the absorber oil which is about 98.5% hexane and heavier. The process is an improvement on conventional processes using an absorption oil which is stated to be a moderately high boiling hydrocarbon liquid having a molecular weight range of 150-300 and a normal boiling range of 350.degree.-500.degree. F., this oil being passed through the absorption zone at rates of about 10-100 gallons per thousand cubic feet of feed gas. Because this feed rate is relatively high, it invariably causes lean gas product contamination and consequently requires periodic decontamination in a separate distillation operation. The products of this process are a dry gas containing methane and ethane and a liquefied product containing as high as 95% of the propane together with the C.sub.4 +constituents.
U.S. Pat. No. 2,868,326 discloses a process for treating a hydrocarbon gas containing hydrogen sulfide with an absorption oil in a deethanizing absorber and in a propane absorber. The absorption oils may each be a gasoline fraction or may be a straight run oil (400.degree. F.E.P.).
U.S. Pat. No. 2,938,865 describes an extractive distillation process for treating a hydrocarbon gas, such as a compressed, wet gas obtained as the overhead after fractionating the gas produced by catalytic cracking of a gas oil, the absorption oil having a boiling range which is close to the boiling range of the material being separated from the feed material. A suitable absorption oil for use in a deethanizing absorber is an unstabilized gasoline having an end point of about 400.degree. F. and containing some butane.
U.S. Pat. No. 3,236,029 relates to recovery of propane from the overhead gas stream of a deethanizer still which uses a lean absorption oil, such as a mineral seal oil, without need for use of a propane-ethane fractionating column. The rich oil from the absorber is flashed to remove methane, heated, and fed to the upper portion of the stripping section of an extractive distillation column used as the deethanizing absorber. Absorption oil from a stripping still is fed to the top of the absorber section of the same column.
U.S. Pat. No. 3,287,262 describes a process for treating raw or wet natural gas to recover therefrom gasoline-boiling range hydrocarbons, particularly C.sub.3 -C.sub.6 hydrocarbons. The natural gas is fed to the bottom of an absorber which also receives at its top a presaturated lean oil at 6.degree. F. The rich oil from this absorber is fed at -20.degree. F. to the midsection of an extractive distillation column used for deethanizing. This column has a pre-saturator at its top above its upper section, in which the ethane is absorbed in the lean oil as the gases rise from the lower section, thereby preparing a feed for the absorber column. A lean absorption oil is fed to the top of the pre-saturator for this purpose. A separate stream of lean oil is also fed to the top of the upper section, beneath the pre-saturator.
U.S. Pat. No. 3,907,669 provides lower energy consumption in a process for the separation and recovery of desired liquid and vapor constituents from a feed stream containing them. The feed stream is contacted with the lean absorption oil in an absorption column, and the resultant rich oil is passed to a stripping column having a reboiler and then to a fractionation column. A portion of the stripped oil is withdrawn from the stripping column and a balance of the absorption oil from the fractionation column. Energy consumption is decreased because the portion of absorption oil withdrawn from the stripping column does not pass through the fractionation column.
U.S. Pat. No. 4,009,097 relates to a process for recovery of selected hydrocarbon liquid and vapor constituents from a feed stream by countercurrent absorption with primary and secondary lean oils in an absorption zone, stripping the rich oil from the bottom of the zone, passing the stripped oil to a fractionation zone, mixing the stripped vapor with the feed stream, cooling this mixture to cause partial condensation thereof, separating the cooled mixture into liquid and vapor phases, introducing the vapor phase into the bottom of the absorption zone, and introducing the liquid phase into the absorption zone at a higher point, introducing the mixture of vapor and feed into the absorption zone, returning a portion of the fractionation bottoms to the absorption zone as the secondary lean oil, and returning a portion of the stripped oil to the absorption zone as the primary lean oil.
U.S. Pat. No. 4,368,058 describes a process for controlling the flow of lean absorption oil to the absorption section of an absorber column by measuring the pressure drop across an absorption section of the column. A portion of a vapor is condensed and the oil thus produced is employed as the absorption oil to recover a more readily absorbed portion of the vapors and thus produce a rich oil. Flow of lean oil can be maximized, short of flooding, responsive to the measured pressure differential.
A new selective solvent process has recently been available for the extraction of hydrocarbon liquids from natural gas streams. This process, known as the Mehra Process, utilizes a preferential physical solvent for the removal and recovery of desirable hydrocarbons from a gas stream. In the presence of a selected preferential physical solvent, the relative volatility behavior of hydrocarbons is enhanced. The selected solvent also has high loading capacity for desirable hydrocarbons.
If hydrocarbons heavier than methane, such as ethane, ethylene, propane, propylene, butanes, etc., are present, they can be selectively removed from the gas stream as a combined liquids product by using the Mehra Process. The hydrocarbon component recoveries can be adjusted to any degree varying in the range of 2-98+% for methane, 2-90+% for ethane, and 2-100% for propane and heavier hydrocarbons.
In the Mehra Process, methane is generally considered to be one of the undesirable hydrocarbons which leaves the process as residual gas. However, as taught in U.S. Pat. No. 4,526,594, the residue gas can be selectively purified to become the product gas. The Mehra Process accordingly provides flexible recovery to a selected degree of only economically desirable hydrocarbons as a hydrocarbon liquids product or as a product gas.
Even though the Mehra Process was developed with the viewpoint that economical operation was essential, the superior absorbing qualities of preferential physical solvents, such as mesitylene, compared to "lean oils" were unquestioned. Such absorption oils are truly lean only between the bottom of the regenerator column and the top of the absorption column. They should accordingly be described as lean "lean oil" or rich "lean oil"; preferably, the term, "absorption oil", should be employed.
According to the "Engineering Data Book", Vol II, Sections 17-26, Tenth Edition, 1987, published by the Gas Processors Suppliers Association, 6526 East 60th Street, Tulsa, Ok. 74145, absorption is one of the oldest unit operations used in the gas processing industry. For a given gas, the fraction of each component in the gas that is absorbed by an oil is a function of the equilibrium phase relationship of the components and lean oil, the relative flow rates, and the contact stages. The phase relation is a function of pressure, temperature and the composition of the lean oil.
As components are absorbed, the temperature of the gas and oil phases increases due to heat of absorption. The heat released is proportional to the amount of gas absorbed. In many cases, side coolers are used on the absorber to limit the temperature rise and aid in absorption.
Lean oil typically has a molecular weight in the 100 to 200 range. For ambient temperature absorbers, a heavy lean oil of 180 to 200 molecular weight is normally used. For refrigerated absorbers, a lighter lean oil of 120 to 140 molecular weight is used. A lower molecular weight lean oil contains more moles per gallon, resulting in a lower circulation rate. However, a lower molecular weight lean oil will have higher vaporization losses.
The stripping column is operated at low pressures and high temperatures. Refrigerated lean oil plants normally use direct fixed heaters to vaporize a portion of the rich oil in the stripper (still) to provide the necessary stripped vapor.
In a Summary Report by Grant M. Wilson et al, entitled "K-Values in Absorber Oil-Natural Gas Systems, Experimental Study", Sept. 5, 1968, from P-V-T, Incorporated, P.O. Box 36272, Houston, Tex. 77036, for the Natural Gas Processors Association, 808 Home Federal Building, 404 South Boston, Tulsa, Ok. 74103, the results of 34 tests on two absorption oils were given, according to the following outline:
______________________________________ Series I Series II ______________________________________ Temperature, .degree.F. +40, 0, -40 +40, 0, -20, -40 Pressure, Psia 500, 1000, 1500 500, 1000, 1250, 1500 Composition to absorber top, absorber bottom simulate absorber bottom Absorber oil 103 molecular weight 130 molecular weight ______________________________________
Supplemental data on the 130 molecular weight lean oil are as follows:
______________________________________ FRACTIONAL DISTILLATION ______________________________________ IBP-221.degree. 2.32% 221-250.degree. 3.65% 250-276.degree. 12.27% 276-300.degree. 20.56% 300.degree. + 61.20% 100.00% ______________________________________
______________________________________ Combined Fractional Distillation, Mass Spectrometer and Flame Ionization Chromotographic Analysis Liquid Volume % ______________________________________ Pentane and Lighter Trace* Cyclohexane 0.23 Benzene 0.06 Hexane Paraffins 1.20 Hexane Naphthenes (5 Ring) 0.28 Hexane Naphthenes (6 Ring) 0.44 Toluene 0.43 Octane Normal and iso-Paraffins 8.90 Octane Naphthenes (5 Ring) 1.32 Octane Naphthenes (6 Ring) 4.04 Octane Alkyl Benzenes (Total) 4.74 Distribution Ethyl Benzene 0.54 Para-Xylene 0.67 Meta-Xylene 2.35 Ortho-Xylene 1.18 Nonanes Plus Normal and iso-Paraffins 41.75 Nonanes Plus Monocycloparaffins 26.43 Nonanes Plus Dicycloparaffins 2.80 Nonanes Plus Tricycloparaffins 1.12 Indanes Trace* Naphthalene Trace* Nonanes Alkyl Benzene 4.81 Decanes Alkyl Benzene 1.40 Undecanes Alkyl Benzene 0.05 100.00 ______________________________________ *Trace denotes less than 0.01%
The 103 molecular weight lean oil had the following properties:
______________________________________ Average ASTM Engler Distillation Carbon Tempera- Analysis Number Volume % ture Component Mole % of Cut Over .degree.F. ______________________________________ Propane 0.0028 3 I.B.P. 186 iso-Butane 0.47 4 5 203 n-Butane 1.08 4 10 210 iso-Pentane 1.75 5 20 216 n-Pentane 1.35 5 30 220 Hexanes 3.75 5.92 40 224 Heptanes 21.70 6.98 50 226 Octanes 47.77 7.74 60 229 Nonanes 18.41 8.18 70 232 Decanes 2.75 9.58 80 239 Undecanes 0.95 -- 90 254 Plus 95 278 E.P. 325 Recovery 98.8 Residue 1.2 ______________________________________ PONA Analysis % Specific Gravity ______________________________________ Aromatics 12.2 60.degree. F./60.degree. F.: Naphthenes 39.8 0.7405 Paraffins 47.5 Density at 1500 psia and 75.degree. F.: Dicycloparaffins 0.4 0.7398 g/cc ______________________________________
The 130 molecular weight lean oil had the following properties:
______________________________________ Average ASTM Engler Distillation Carbon Tempera- Analysis Number Volume % ture Component Mole % of Cut Over .degree.F. ______________________________________ Propane 0.0061 3 I.B.P. 242 iso-Butane 0.0092 4 5 270 n-Butane 0.013 4 10 283 iso-Pentane 0.023 5 20 294 n-Pentane 0.017 5 30 300 Hexanes 0.45 5.92 40 306 Heptanes 2.09 6.91 50 312 Octanes 10.20 7.71 60 319 Nonanes 32.96 8.71 70 328 Decanes 34.00 9.64 80 340 Undecanes Plus 20.23 -- 90 361 95 386 E.P. 435 Recovery 98.6 Residue 1.4 ______________________________________ PONA Analysis % Specific Gravity ______________________________________ Aromatics 11.9% 60.degree. F./60.degree. F.: Naphthenes 35.6 0.7703 Paraffins 52.2 Olefins 0.3 Molecular Weight 130 ______________________________________
For both oils, 50% of the total area of the chromatograph was cut on a apiezon column at 320.degree. F. All components were assumed to be paraffins in averaging calculations for the Average Carbon Number of Cut.